Rechargeable batteries such as nickel metal hydride rechargeable batteries and lithium-based rechargeable batteries are widely used as power sources for portable devices such as portable telephones, notebook type personal computers, video cameras, and the like. Nowadays, further miniaturization been eagerly sought in such electronic appliances. In order to minimize the volume occupied by batteries in the devices, the batteries should be in a prismatic shape from the viewpoint of efficient use of available space. However, while offering excellent space efficiency, a prismatic battery has the following disadvantages as compared to a cylindrical battery.
FIG. 5 is a partially cutaway perspective view showing a commonly-used prismatic alkaline rechargeable battery, and FIG. 6 is a semi diagrammatical, partially sectional perspective view showing a commonly-used cylindrical alkaline rechargeable battery.
In the cylindrical battery, elements for electromotive force are constructed by winding a sheet of positive electrode 7 and a sheet of negative electrode 9 with an intervening separator 8 therebetween into a coil, as shown in FIG. 6. In contrast, the elements for electromotive force for the prismatic battery include a plurality of positive electrode plates 7 and a plurality of negative electrode plates 9 stacked upon one another with a plurality of intervening separators therebetween as shown in FIG. 5. Thus the fabrication of the prismatic battery involves far more complicated process steps, resulting in low productivity.
In particular, the separators 8 for the prismatic battery are formed in a bag shape for enveloping one of the positive and negative electrode plates so as to prevent short-circuiting, further causing the manufacturing process to be complicated.
Moreover, it is generally difficult to provide a completely airtight seal between the case 1 and the sealing plate 3 at the open end of the prismatic battery case, leading to another disadvantage that the battery has poorer leakage proof characteristics. A commonly known method of sealing a prismatic battery involves welding the abutment line between the open end edge of the case 1 and the sealing plate 3 by a laser beam. The method inherently requires high cost as it uses laser and is not necessarily promising in terms of reliability because of the difficulty in controlling laser power output to ensure stable welding.
Thus both the prismatic battery and the cylindrical battery have their respective merits and demerits, and a trade-off is desirable between the space utility efficiency of the prismatic battery and its inferiority in producibility and reliability as compared to the cylindrical battery.